FIELD OF THE INVENTION
The present invention relates to a method for determining the moisture content of textile goods in a laundry drier. In general, electric current is passed through the textile goods via electrodes which touch the textile goods and the laundry voltage drop across the textile goods is determined at the electrodes. The conductivity of the textile goods is determined from the laundry voltage. The moisture content of the textile goods is then determined from the conductivity.
When drying textile goods in a laundry drier, it is a major advantage to know the actual moisture content of the textile goods. This information can be used, for example, to determine the remaining time for the drying process, to control the drying process optimally, or to match the heating power to the requirement. Furthermore, if the moisture content is known, the drying process can be ended exactly at the desired residual moisture content. The optimum residual moisture content can be set irrespective of what else is to be done to the textile goods. If, for example, it is intended to iron the textile goods after drying, a higher residual moisture content is advantageous than for storing them.
The moisture content of textile goods in a laundry drier is normally determined by measuring their electrical conductivity. The moisture content is thereby proportional to the conductivity. As a rule, two electrodes are applied to the textile goods for this purpose, one of which is normally the drum in which the textile goods are located. A voltage is applied via a resistor to the two electrodes, and causes current to flow through the textile goods. The laundry voltage drop across the textile goods is measured at the electrodes. The voltage drop is used to determine the conductivity, and the moisture content, of the textile goods.
A disadvantageous feature of this method is that, when a voltage for conductivity measurement is applied between the moist textile goods and the electrodes, a battery element is formed, whose voltage is dependent in particular on the conductivity of the water, the nature and composition of the materials used, and the magnitude and polarity of the voltage applied for measurement. This battery element voltage, which is called the "polarization voltage" in the following text, can be superimposed on the laundry voltage drop across the textile goods, and can thus corrupt the determination of the conductivity. The polarization voltage occurs, in particular, with moist laundry, so that wetter laundry items are more severely affected by it.
German published patent application DE 34 17 482 A1 discloses a method in which the conductivity of the textile goods is measured when they are still wet at the start of the drying process. There is thus also determined, inter alia, a correction factor which is dependent on the laundry drier load and on the conductivity of the water. The correction factor is stored and is used during the drying process to correct the measured conductivity, which is corrupted by various factors. The polarization voltage is thereby also detected and is compensated for. It is disadvantageous that the correction value is determined only at the start of the drying process, so that changes in the polarization voltage during drying cannot be taken into account. Furthermore, the correction value is dependent on a number of influencing variables, which are subject to different changes, so that objective compensation for the polarization voltage is impossible.
Methods are also known from the prior art in which a pulsed d.c. voltage or an a.c. voltage is used to measure the conductivity of the textile goods. The resulting polarization voltage is very low or does not occur at all. These methods have, in particular, the disadvantage that they involve a high level of complexity to produce the specific measurement voltage and to determine the conductivity and the moisture content.